If your car was not manufactured for sale in the United States, the app won't recognize your Vehicle Identification Number (VIN). This means it won't be able to automatically detect it and if you try scanning it or entering it, it won't recognize it. Please see this FAQ topic about why Automatic doesn't support other countries right now.
If your car was manufactured for sale in the U.S.A., please read on...
If you tried scanning your VIN and it didn't work, it's possible you scanned the wrong spot. The windshield VIN doesn't usually have a barcode. You need to scan the VIN on the inside of the driver-side door.
If the app keeps crashing while trying to scan your VIN, check your iPhone settings for barcode scanning as it's possible they're turned off and causing the app to crash at that step. Or you can enter your VIN manually.